Microbead optical sensor with layered plasmon structure for enhanced detection of chemical groups by SERS

ABSTRACT

An optical sensor and method for use with a visible-light laser excitation beam and a Raman spectroscopy detector, for detecting the presence chemical groups in an analyte applied to the sensor are disclosed. The sensor includes a substrate, a plasmon resonance mirror formed on a sensor surface of the substrate, a plasmon resonance particle layer disposed over the mirror, and an optically transparent dielectric layer about 2-40 nm thick separating the mirror and particle layer. The particle layer is composed of a periodic array of plasmon resonance particles having (i) a coating effective to binding analyte molecules, (ii) substantially uniform particle sizes and shapes in a selected size range between 50-200 nm (ii) a regular periodic particle-to-particle spacing less than the wavelength of the laser excitation beam. The device is capable of detecting analyte with an amplification factor of up to 10 12 -10 14 , allowing detection of single analyte molecules.

This patent application is a continuation of U.S. patent applicationSer. No. 12/794,622, filed Jun. 4, 2010, now U.S. Pat. No. 8,093,065,which is a continuation of U.S. patent application Ser. No. 12/059,736,filed Mar. 31, 2008, now abandoned, which is a continuation of U.S.patent application Ser. No. 11/133,632 filed May 19, 2005, now U.S. Pat.No. 7,351,588, which claims priority to U.S. Provisional PatentApplication No. 60/572,959 filed May 19, 2004, now abandoned, all ofwhich are incorporated herein in their entirety by reference.

This work was supported in part by U.S. Government Agency and U.S.Department of Defense Air Force Contract No. AFOSR F49620-03-C-0058. TheUnited States Government may have certain rights in this invention.

FIELD OF THE INVENTION

The present invention in general relates to a novel optical sensorcomposed of SERS-active plasmon particles over a plasmon mirror forenhanced localized optical phenomena, and the use of this effect forultrasensitive chemical and biological sensing with high structuralspecificity and with high detection sensitivity.

References

The references below are cited as part of the background of theinvention and/or as providing methodologies that may be applied tocertain aspects of the present invention. These references areincorporated herein by reference.

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BACKGROUND OF THE INVENTION

A variety of methods for confinement of light and for localization andenhancement of electromagnetic field in nanostructures, for the purposeof enhancing various localized linear and nonlinear optical phenomenaare known in the prior art (See, for example, A. Wokaun, 1984: M.Moskovits, 1985). Most attention in the prior art has been related tothe phenomena of Surface Enhanced Raman Scattering (SERS), based onlocalization and confinement of light near the surfaces of substrateswith nanoscale structure. SERS has proven to be a powerful analyticaltool for ultra sensitive chemical and biochemical analysis (K. Kneipp etal., 1999).

One SERS-based structure that has been proposed employs an opticalstructure composed of a metal island film (MIF) over a smooth metalsurface (H.-G. Binger et al., 1995, G. Bauer et al., 2003). A metalisland film consists of a random two-dimensional array of metalparticles, each of several (typically, 2-10) nm in largest sizedimension. The shapes of the metal particles are also variable, so it isdifficult to characterization the particles structurally. (The particlesform a stochastic array of particles resembling oblate spheroids withall minor axis oriented normal to substrate surface, e.g., glass,quartz, or silicon.) For a variety of reasons that will become clearbelow.

The metal island film MIF is separated from a smooth metal layer by anintermediate spacer layer made from optically transparent dielectricalmaterial, the thickness of which controls the strength of theinteraction between the plasmons localized on the islands and thesurface plasmons of smooth metal layer. The metal particles (islands)can be thought of as nanoscopic antennas, collecting the incidentradiation and then transferring the energy into the nearby gap modes,that may be trapped into guided modes propagating in all directions inplane of surface (omnidirectional coupling). The ability of structure toabsorb light at specific wavelength depends on the existence of anoptimal spacer layer thickness that will maximize absorption instructure for specific wavelength close to that of excitation light(Leitner et al., Appl Opt 1993; W. R. Holland et al., 1983, T. Kune etal., 1995). For a variety of reasons that will become clear below, themaximum enhancement achievable with such MIF structures is limited tobetween about 10⁶-10⁸.

The phenomenon of interaction of localized plasmons (LP) with surfaceplasmon polaritons (SPP) in plasmon materials has been discovered andnew method of excitation of SPP in plasmon resonant smooth filmsmediated by nanoparticles has been proposed (S. Hayashi et al., 1996).An interesting phenomenon associated with SPP excitation is thegeneration of a strong electromagnetic field near the metallic surface.It is a generally accepted mechanism that a strong electromagnetic fieldleads to enhancement of various linear and nonlinear optical processesnear the surface via a mechanism of surface-enhanced spectroscopy (M.Moskovits, 1985; G. C. Schatz and R. P. Van Duyne, 2002). According tothis mechanism, the enhancement of SERS signal is proportional to E⁴,where E is electromagnetic field near metal surface.

One typical application of this phenomenon is the surface enhanced Ramanscattering of molecules adsorbed on metallic surfaces that supportplasmon resonances at both the excitation and scattering wavelengths.Typical enhancement achieved by using electrolysis roughened silver orby using substrate prepared by nanosphere lithography (J. C. Hulteen etal., 1999) is in the range 10⁶-10⁸. In general, the degree ofenhancement seen is not uniform across the sensor nor reproducible.

The inability to control parameters of MIF metal surface and intrinsiclimitations in size of metal particles to less than 5 nm (V. Matyushin,A et al., 2004) precludes their use for SERS (H.-G. Binger et al., 1995)limits the sensitivity of such a system since MIF-metal substratestructures do not have strong enhancement of Raman signal. ThereforeMIF-metal substrate have been reduced to practice only for enhancementof fluorescence in so called “resonant nanocluster biochip” technology(G. Bauer et al., 2003; T. Schalkhammer et al., 2003).

SUMMARY OF THE INVENTION

The invention includes, in one aspect, an optical sensor for use with avisible or near infrared (NIR) laser excitation beam and a Ramanspectroscopy detector, for detecting the presence of chemical groups inan analyte applied to the sensor. The sensor includes a substrate, aplasmon resonance mirror formed on a sensor surface of the substrate, aplasmon resonance particle layer disposed over the mirror, and anoptically transparent dielectric layer about 2-40 nm thick separatingthe mirror and particle layer. The particle layer is composed of aperiodic array of plasmon resonance particles having (i) a coatingeffective to binding analyte molecules, (ii) substantially uniformparticle sizes and shapes in a selected size range between 50-200 nm(ii) a regular periodic particle-to -particle spacing less than thewavelength of the laser excitation beam. The particles may have highsymmetry or reduced symmetry shape, and more generally, as will beconsidered below, may be spherical, spheroid, rod like, cylindrical,nanowire, tubes, toroid, or other shapes that, when uniform, can bearranged with regular periodicity. A particle layer, as defined herein,is also intended to encompass a regular array of holes in a planarplasmon layer, where the holes have the dimensions set out above for theparticles. The device is capable of detecting analyte with anamplification factor of up to 10¹²-10¹⁴, allowing detection of singleanalyte molecules.

The mirror may be a silver, gold or aluminum layer having a layerthickness between about 30-500 nm. The particle have a preferreddimension in a selected size range of between 50-150 nm, and may beformed from silver, gold, or aluminum solid or particles having a shellformed of such metals. In an exemplary embodiment, the mirror andparticles are either both gold or both silver, and the particles aresubstantially spherical.

The particle layer may be formed of a regular array of closed packedplasmon resonance particles having a particle-to-particle spacing ofabout 20 nm of less, including direct particle-to-particle contact. Theparticle layer may include a periodic array of at least 50 particles inat least one direction, preferably at least 50 particles in each of twoplanar directions, e.g., orthogonal directions or directions diagonaldirections dictated by close packing. The sensor may include one or moreadditional particle layers, each separated from the immediatelyunderlying particle layer by an optical dielectric layer having athickness of between 2-40 nm. The substrate may have a planar or curvedshape, e.g., when formed on spherical beads or inside pores in a porousfilter.

In another aspect, the invention includes a method of detecting chemicalgroups in an analyte with an amplification factor of at least 10¹⁰. Inpracticing the method, molecules of analyte are bound to plasmonresonant particles in the particle layer of an optical sensor of thetype described above, the sensor surface is irradiated with a visible orNIR laser beam, and the Raman spectrum produced by the irradiating isrecorded. The method may be effective to produce an amplification factorof at least 10¹², and therefore capable of detecting chemical groups inone or a small number of analyte molecules. The method allows Ramanspectrum analysis at an irradiating beam power as low as 1-100 μW (microW).

These and other objects and features of the invention will be more fullyunderstood when the following detailed description of the invention isread in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the arrangement of components of a basic planar structurefor confinement, localization, and enhancement of EM field according toone embodiment of the invention, and illustrates how it is used formeasurement of SERS spectra.

FIGS. 2A and 2B show schematically the structure of GMs and SPPs in thesame embodiment and illustrates in general how key principal mechanismof invention works.

FIGS. 3A and 3B shows an embodiment of the invention in which theperiodic structure is a 2-dimensional array of nanosize holes inmetallic film.

FIGS. 4A and 4B shows an embodiment of the invention in which theperiodic structure is a 2-dimensional array of nanosize tubes imbeddedin metallic film.

FIGS. 5A through 5D show an embodiment of the invention in which theperiodic structure is a metallic grating consisting of a one-dimensionalarray of metallic strips or cylinders.

FIG. 6 is an AFM topographic image of a 2 micron by 2 micron area ofsurface of a planar SERS-active substrate fabricated according toExample 2. The image demonstrates the uniformity and high density ofpacking of nanoparticle placement on the surface.

FIGS. 7A-7C show various aspects of an experimental set up with a Ramanmicroscope and fluidic cell used for measurement of SERS spectra fromliquid samples.

FIG. 8 shows a SERS spectra for Rhodamine 6G (R6G) molecules obtained ina fluidic cell using a Raman microscope Horiba-Jobin-Yvon Lab Ram HR 800and Argon laser.

FIG. 9. shows a SERS spectra of Rhodamine 6G molecules obtained influidic cell using Raman microscope.

FIG. 10A is a Raman image of 20×20 μ area for main intensity peak (1280and 1400 cm-1) of Rhodamine 6 G molecules with a baseline correction in%.

FIG. 10B is a SERS spectra of Rhodamine 6 G molecules at maximum andminimum intensity with baseline correction.

FIG. 11A is a Raman imaging map of Rhodamine 6G (R6G) molecules on SERSslide of area 20×20 micron. Dotted lines present spots from which Ramanspectra have been collected.

FIG. 11B is a SERS spectra of Rhodamine 6G molecule along Line 1 out of21, from the top of map demonstrating uniformity of “hot spots” acrossthe surface of substrate;

FIGS. 12A through 12D illustrate the use of SERS-active structure of thepresent invention integrated into a filter based optical SERS sensorwith a planar (12A and B) and nonplanar (12C and D) SERS-active surface.The filter is made of an optically transparent porous silica. Part ofthe internal surface of the porous material is covered by the resonantSERS-active structure of the present invention.

FIGS. 13A and 13B show a diagram of a fiber-optic coupled optical sensorfor remote detection and identification of environmental contaminantsand hazardous materials;

FIGS. 14A and 14B show a diagram of a microbead-based optical SERSsensor with a nonplanar spherical SERS-active surfaces;

FIG. 15 Illustrates the use of a bead aerosol to detect distantlybiological and chemical warfare agents and explosives with a Ramanstandoff system such as LIDAR;

FIG. 16 Illustrates an embodiment of a planar microfluidic optical SERSsensor, in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A. Definitions

The terms below have the following meaning, unless otherwise indicated.

“Plasmon resonant metal” includes any metal, such as gold, silver, oraluminum which can support surface electromagnetic modes—surface plasmonpolaritons (SPP), which are coupled modes of photons and plasmons.

“Chemical group” in a sample may include subunits in a polymer, orsubunit moieties, such as nucleic acid bases, or chemical constituentgroups, such as hydroxyl, amine, alkyl, acid, or aldehyde groups. Suchchemical groups are characterized by a unique enhanced Raman spectralsignatures or features.

“Gap modes” or “GMs” refer to electromagnetic normal modes orelectromagnetic eigenmodes that are excited by external electromagneticfield in a space between two or more plasmon resonance particles andwhen plasmon resonance particles are placed near (less than 40 nm) ametal surface, preferably a plasmon resonant metal surface.

“Plasmon resonance particles” (PRPs) are particles are particles formedof a plasmon-resonance metal, such as gold, silver, or aluminum, orparticles having a shell of such metal. In the present invention, PRPhave their largest dimension typically in the 50 nm to 200 nm sizerange.

“Gap-mode enhanced Raman spectrum” of a sample refers to spectralfeatures in a Raman spectrum of the sample that are enhanced by thepresence of gap modes at the sample. “Photonic crystals” refers to 1-,2-, 3-dimensional structures with periodic distribution of refractionindex that results in a band-gap structure, with the result that photonswith energies corresponding to this band gap cannot propagate onphotonic crystal and may exist only in localized state.

“Photonic band gap” refers to a range of energy of photons in which theycannot propagate in photonic crystal structures.

“Visible light” refers to the portion of the electromagnetic spectrumthat is visible to the human eye, generally in the wavelength rangebetween 400 nm to 700 nm range.

“Near infrared” refers to the portion of the electromagnetic spectrumwith a wavelength longer than visible light, but shorter than microwaveradiation, generally in the wavelength range between 700 nm and 1 mm.

B. General Description of the Invention

The present invention provides a plasmon resonance nanostructure thatallows precise control and tunability of its optical response throughplasmon resonance effects. This is achieved by one or more periodicplasmon layers operating as 2-D or 3-D photonic crystals withappropriate photon band gap structure enhanced by coupling to a plasmonmirror through an optically transparent dielectric layer having aselected thickness of less than about 40 nm.

The general design of structure according to concept of the invention,which will be referred to “periodic plasmon nanostructure over plasmonmirror” consists of a continuous plasmon resonant material referred toas a “plasmon mirror” and at least one particle layer consisting of a1-D or 2-D periodic array of plasmon resonance particles (or otherregular nanostructures, as discussed below) in which localized plasmons(LPs) may be excited. Plasmon resonance coupling between the particlelayer and mirror is through a selected-thickness, optically transparentdielectric layer having a selected “tuned” thickness between about 2-40nm, preferably 2-20 nm.

The particles forming the particle layer are substantially uniform insize and shape, in a selected size range between about 50-200 nm,preferably 80-150 nm, depending on the excitation wavelength. Theparticles may have high symmetry or reduced symmetry shape, and moregenerally, as will be considered below, may be spherical, spheroid, rodlike, cylindrical, nanowire, tubes, toroid, or other shapes that, whenuniform, can be arranged with regular periodicity. They may behomogeneous consisting from one material—silver, or gold, or fromcomposite such as nanoshells (J. West et al., “Metal Nanoshells forBiosensing Applications”, U.S. Pat. No. 6,699724, Mar 2, 2004.). Theperiodicity of the particle layer(s), i.e., the spacing between adjacentparticles in any direction, may vary from a close-packed arrangement, inwhich the particles are separated from one another by a spacing ofbetween particle size plus 0-20 nm, or with a periodic spacing up to thewavelength of incident light, with optimal coupling and enhancement ofsignal being observed in the close-packed arrangement, preferably withspherical particles. A particle layer, as defined herein, is alsointended to encompass a regular array of holes in a planar plasmonlayer, where the holes have the dimensions set out above for theparticles. The particles in the particle layer are separated by orembedded in a dielectric material which may be air or a solid, opticallytransparent dielectric material, such as like that forming thedielectric layer.

The plasmon resonance response of the nanostructure is tunable and maybe controlled by adjustment of the parameters of the nanostructureincluding the spacing between layers, the size and shape of thenanoparticles, the spacing between nanoparticles, the periodicity of theparticles forming the particle layer, and, and the dielectric constantand thickness of the dielectric layer. Maximum localization andenhancement of EM field is achieved when the frequency of the excitationlight is the same as or close to the frequency of plasmon resonance ofthe nanostructure as a whole, or more precisely, the plasmon resonancefrequency should be between the frequency of incident light and that ofscattered light. Plasmon resonance frequency and shape of plasmonresonance response in such complex metal-dielectric nanostructuredepends on many parameters (size, material, shape of nanoparticles, andtheir arrangement with respect to each other and with respect to plasmonmirror surface). However, strongest plasmon responses are obtained ondipole plasmon resonance excitations of LPs on isolated nanoparticles.Maximum confinement and localization and enhancement of the EM field inthe structure is achieved through a mechanism of excitation of gapelectromagnetic modes (GMs) or eigenmodes of the particle layer, andsurface plasmon polariton modes (SPPs) excited on the smooth surface ofthe mirror. This mechanism operates through coupling and interactionsbetween these modes and between the electromagnetic field of theexcitation light.

An additional advantage of regular array of LP oscillators over acontinuum of SPP under condition of coupling between them through GMs(close proximity between two layers separated by dielectric) is themechanism of synchronization of LPs through SPPs that result in thenarrowing of plasmon resonance and additional dramatic enhancement oflocal field and corresponding Raman signal. However, this effect existsin relatively narrow range of spectra. Typically the narrow collectiveplasmon resonances are in a range of 450-800 nm, but best enhancementachieved in range 500-600nm for silver NP and 600-750 nm for gold NP.

A general advantage of periodic regular array in the particle layer isthat it now has both high plasmon resonant response and properties of aphotonic crystal that result in additional effect of focusing andconfinement of incident light beam due to confinement in photon band gapstructure. This is in contrast to a random array of LPs over SPPcontinuum, where both effects synchronization between LP and focusing ofincident light beam (by mechanism of Anderson localization disclosed inD. Wiersma, “Localization of light in a disordered medium”, Nature, 390,671-673 (1997)) are present, but overall the effect of EM fieldenhancement is significantly less, since the density of “hot spots” isrelatively small. According to the generally accepted paradigm of SERS(M. Moskovits, 1985: G. Schats et al., 2002), enhancement of Ramansignal happens through local field enhancement due to plasmon excitationin so called “hot spots.” The general structure of “hot spots” indifferent array structures is explained and illustrated on FIGS. 2-5below. From a practical stand point, this enhanced interaction from aperiodic particle layer and plasmon mirror allows for a highlyreproducible high quality Raman spectra with extremely low excitationpower (typically 10-100 microwatt, and less than 1 microwatt for somesamples).

The plasmon resonance nanostructure of the invention may be used in avariety of applications in analytical instrumentation, analyticalchemistry and spectroscopy. As an example it may be used as a substratein mass spectrometry devices for improvement of Laser DesorptionIonization, such as MALDI-TOF, SELDI-TOF). Another major field of use isenhancement of a variety of localized linear and nonlinear opticalphenomena such as Generation of Harmonics, Coherent Anti-Stokes Ramanscattering (CARS) and in particular as SERS-active substrate.

In particle, the nanostructure of the invention may be used forenhancement of Raman signal in various optical devices and opticalsensor devices. In particular, one important practical application ofthe invention is its use as a SERS-active sensor for real-time alloptical ultrasensitive detection and identification of chemical groupsin chemical and biological analytes in samples in solid, liquid andgaseous environment. Four major embodiments of optical devices andoptical sensors using these fundamental interactions are discussed belowin Section D1-D4, and include:

1. Optical devices and sensors with planar SERS-active surfacesaccording to present invention (SERS-based), implemented, for example,in microfluidic chip platform;

2. Filter based optical devices and sensors with nonplanar SERS-activesurfaces (SERS based) from optically transparent porous and mesoporousmembranes and materials with all or part of internal surfaces covered bythe resonant structure of present invention. This sensor is especiallyuseful for continuous monitoring of environmental contaminants in liquidand gaseous phase;

3. Fiber optic coupled optical devices and sensor with both planar andnonplanar SERS-active surfaces (SERS based) for distant sensing(detection and identification) of environmental contaminants andhazardous materials; and

4. Mirobead based optical devices and sensors with nonplanar (sphericalor spheroid shape) SERS-active surfaces (SERS based)—possible use are inmicrofluidic flow as well as in aerosol samples.

C. Basic Optical Sensor of the Invention

The structural requirement in the optical sensor of the invention can beunderstood from the following basic description of the physicalinteractions responsible for the giant EM enhancement it provides. Underplasmon resonance conditions, corresponding to plasmon oscillation ofindividual NPs, the EM field excites LP oscillation on each particle.For silver NPs in the range of 50-150 nm, plasmon resonance frequency isin a range 460-520 nm. This geometry of excitation is also optimal forexcitation of two types of Gap electromagnetic modes (GMs). The firsttype is the GM between adjacent NPs in the layer array, and the secondtype, between NPs and the plasmon mirror surface. For efficientexcitation of GMs, the spacing between adjacent particles (theperiodicity of the layer) should be regular and less than wavelength ofthe EM field in the dielectric media (typically is 250-700 nm, sincedielectric constant of transparent matrix and spacer layer is in a range1.5-2.5), but best results are for a close-packed arrangement having aperiodicity close to the diameter of the NPs plus up to 20 nm.

If the NP array (particle layer) is in close proximity to the plasmonsurface (a distance less than about 40 nm), then SPPs in the mirror areexcited and propagate in all directions in the surface plane. Due tocoupling with the LPs of nanoparticles, SPPs creates a new mechanism (inaddition to EM wave) of long range interaction between LP oscillations.Long range interactions produce synchronization of phases of LPoscillations in the NP array and results in narrowing bandwidth ofplasmon resonance, so called collective plasmon oscillation. Optimalparameters for such synchronization to occur are: NP sizes in range50-200 nm preferably 80-150 nm, and a regular periodicity(particle-to-particle spacing), preferably in both directions in theparticle layer, of less than the wavelength of the excitation light, andpreferably a close packed arrangement having a periodicity of the NPsize plus up to 20 nm. Best amplification is achieved for a perfectperiodic array with a number of NPs along one dimension of more than 50.Any deviation from perfect periodicity and from uniformity in NP sizewill reduce the enhancement effect since it results in disruption ofsynchronization and broadening of plasmon resonance shape. This explainswhy random arrays and fractal structures from NP are less efficient thanthe periodic nanostructure over plasmon mirror disclosed in theinvention.

As an example of an exemplary nanostructure constructed in accordancewith the invention, reference will be made to the optical sensor shownin FIGS. 1 and 2. The structure consists from substrate 10 providing anupper sensing surface. The substrate may be any dielectric support, suchas glass, ceramic, or silicon waver slide or waver. Formed on thesensing surface of the substrate is a plasmon resonance mirror 20 whichis formed from a material, such as silver, gold, or aluminum, capable ofsupporting surface plasmon polaritons (SPPs). This layer can be createdby standard vacuum deposition technique (e.g., V. Matyushin, A et al.,2004). The thickness of the layer could be in a range 20-500 nm or moreas long as it can function as a mirror surface in the optical range ofspectra.

Spacer layer 30 formed over the mirror is composed from opticallytransparent dielectric material, for example, LiF formed by vacuumdeposition, or dried polymer films, as described below. The thickness oflayer is in a range less than 50 nm, preferably less than 40 nm, andmore preferably 3-20 nm, e.g., 5-25 nm. If a self-assembling method isused for making the layer of nanoparticles on the dielectric layer, thelayer is preferably formed of a polyamine or the like capable of formingcovalent chemical bonds to the particles (and with the mirror layer).The dielectric spacer layer can be produced with a controlled thicknessby using, for example, a micromachined piezo driving system. In thiscase, the optical plasmonic properties of the substrate can bedynamically controlled to allow optimizing absorption maxima.

A nanoparticle layer 40 may be formed, for example, by a method of selfassembling (B. E. Baker et al., 1996), which allow plasmon particles ofany size (e.g., 80-100 nm sizes), employing particles with highuniformity in shape and size. Composite (Gold-Silver or Silica-SilverShell) nanoparticles may also be used, as may low symmetry nanoparticlessuch as ‘nanobowls’ (Y. Lu et al., 2004). By using template directedself assembling techniques (Y. Xia et al., 2003) perfectly ordered(cubic or hexagonal or other symmetry) arrays of particles can becreated with controlled surface density and interparticle distances.Plasmon particles can be covered by a protective layer individually.

A protective coating layer 50 may be, for example, formed from SiO orother dielectric optically transparent material. In an embodiment inwhich the particles have individual protective coating, a protectivelayer is not necessary. The thickness of the protective layer is lessthan 5 nm, preferably less than 2 nm. The protective layer or thecoating on the individual particles may be derivatized with analytebinding molecules, such as antibodies, ligands, DNA fragments, and thelike, or analyte binding to the coating or protective surface may be bynon-specific absorption. In some embodiments, individual particles maybe coated by a molecular imprinted polymer (MIP) to bind specific targetanalyte (K. Haupt, “Imprinted polymers-Tailor-made mimics of antibodiesand receptors”, Chem. Comm., 2003, 171-178) or by monoclonal antibodiesfor specific analytes. In either case, the surface of the sensor isexposed to analyte under conditions in which analyte molecules bind tothe coating surface, typically placing the analyte within 0-5 nm from aPRE in the particle surface. However in some cases when analytemolecules may penetrate and bind directly to particle surfaceenhancement may be even larger. The figures show analyte molecules 80placed on surface of coating 50.

In its optical sensing mode, the sensor surface is irradiated with avisible or NIR laser beam 60 through a focusing lens 70. As shown inFIGS. 2A and 2B, the incident light, indicated at 110, excites Gap Modes130 (GMs) localized presumably within the particle layer and between theparticles and the plasmon mirror, and gap modes 140 betweennanoparticles (NPs) 100 forming the particle layer. Although not shown,localized plasmons (LPs) are formed about each particle. Surface plasmonpolaritons (SPPs) formed on the surface of the metal film are shown at150. The sinusoidal wave representation of the SPPs is intended toindicate that the SPPs are propagating, and not stationary. As seen, theGMs produce extremely high local electric field in close proximity tothe particle surfaces. An end enhanced EM field results in enhancementof a Raman cross section that scales as E⁴ (M. Moskovits, 1985: G. C.Schatz, and R. P. Van Duyne, 2002). Enhanced Raman signal light,indicated at 120, is generated by analyte molecules is collected inbackscattering arrangement and is send to dispersive element of Ramanspectrometer detector (not shown), where spectra of substance areanalyzed and information about chemical groups is identified.

FIGS. 3-5 demonstrate other embodiments of “plasmon lattice over plasmonmirror” structure operating according to same general principle of workas described above. For example, in FIGS. 3A and 3B, the 2-D periodicplasmon structure is a metallic film 20 a with a periodic array ofnanoholes 102 with diameters in the range 20-200 nm and spacing betweenholes in a range less than wavelength of incident light. Between theplate with nanoholes 20 b and a plasmon mirror 20 a, there is dielectriclayer 30 with thickness in the range 2-40 nm. Incident electromagneticwave 110 excites LPs on the surface of each nanohole and SPPs 150 on thesurface of the metal film. Due to resonance effects of anomaloustransmission of light through array of subwavelenth nanoholes (T.Ebessen et al., Nature, 391, 667, 1998) electromagnetic field penetratesinto the volume between the plasmon mirror and array of nanoholes andexcites GMs 132 and two types of SPPs (shown at 150) in the surface ofthe plasmon mirror and on both surfaces of metal films with the array ofnanoholes 20 b. The SPPs and GMs interact with each other through thedielectric layer 30 of less than 40 nm thickness. Due to close proximitythis additional long range interaction between SPPs and LPs stimulatesynchronization of phases of LP oscillations in array and as a resultplasmon resonance gets narrowed and local field on surfaces of NPsubstantially enhanced.

The nanohole lattice structure shown in FIGS. 3A and 3B may be formed,for example, by using photolithographic etch techniques to form a silveror gold layer containing an array of holes, each hole having a selecteddiameter in the 50-200 nm range, a firm thickness in the range 20-200nm, and a hole-to -hole spacing in the range of up to the excitationwavelength and preferably in the range of hole diameter up to 20 nm.This film, once formed, can then be transferred to a structurecontaining the substrate mirror layer and dielectric layer to form theoptical sensor nanostructure of the invention.

Analyte molecules 82 on the lattice layer may be adsorbed on the surfaceinside or near nanohole 102 and became exposed to strongly enhancedlocal field of NPs. Due to the SERS effect described above, Ramanscattered signal 130 is enhanced, and this signal is detected by anoptical system and subjected to spectral analysis in Raman spectrometerdevice.

FIGS. 4A and 4B illustrate similar embodiments to that presented onFIGS. 3A and 3B, except that the geometric parameters of the plasmonarray lattice consist of metallic film 20 b with sub-wavelength sizeholes and nanotubes 104 attached to each hole. The lattice period inthis case has same range as that discussed, namely less than wavelengthof the excitation light. The geometric structure of GMs between thelattice layer and mirror (shown at 134) will be slightly different withthis configuration; however, the fundamental mechanism of interactionthrough excitation of LPs, GMs and SPPs and the effect ofsynchronization in an array of nanostructures is basically the same.

The nanotube lattice structure shown in FIGS. 4A and 4B may be formed,for example, by using self-assembly techniques to form an assembledarray of sliver or gold nanotubes which are then transferred to astructure containing the substrate mirror layer and dielectric layer, toform the optical sensor nanostructure of the invention. Alternatively,the nanotube layer that is transferred to the mirror structure can beformed by photolithographic techniques in which both the tubes and thetube interiors are produced by etching of photoactivated regions of thearray. In this embodiment, each tube has a selected ID in the 50-200 nmrange, a film thickness (tube length) in the range 20-200 nm, and atube-to-tube spacing in the range of up to the excitation wavelength andpreferably in the range of tube diameter up to 20 nm.

It is worth noting that due to the symmetry of the lattice in FIGS. 2-4,the excitation of SPPs is omnidirectional and therefore the efficiencyof excitation does not depend on polarization of incident light underperpendicular incidence geometry.

An alternative embodiment of the “plasmon lattice over plasmon mirror”structure is presented on FIGS. 5A-5D, which illustrates a structure inwhich the plasmon lattice is a 1-D array of nanocylinders 106 in FIGS.5A and 5B or nanostrips 106 in FIGS. 5C and 5D which form, in effect, aplasmon metal grating as a lattice. (Other structures common to thoseshown in FIGS. 2-5 are identified with the same numerals in all of thesefigures). The range of geometric parameters is the same as in previousexamples. All geometrical dimensions of the structure, including thediameter of the cylinders or strips, and the periodicity of the surfacestructures are less than wavelength of light. Specifically, eachcylinder or strip has a selected OD or width in the 50-200 nm range, andthe spacing between cylinders or widths of the strips is such as to givea periodicity of up to the wavelength of the excitation light andpreferably the range cylinder OD (or strip width) plus up to 20 nm.

The nanocylinder lattice structure shown in FIGS. 5A and 5B may beformed, for example, by using self-assembly techniques to form anassembled array of sliver or gold nanocylinders which are thentransferred to a structure containing the substrate mirror layer anddielectric layer, to form the optical sensor nanostructure of theinvention. Alternatively, the nanocylinder layer that is transferred tothe mirror structure is formed by photolithographic techniques.

The mechanism of operation through cylinder-to-cylinder GMs, indicatedat 146, and cylinder-to-mirror GMs, indicated at 134, and SPPs,indicated at 150, are substantially the same as above. However, due tothe reduced symmetry in 1-D, the excitation efficiency now depends onthe orientation of polarization vector in lateral plane. More efficientexcitation of GMs is achieved if the direction of the electric field inEM wave is perpendicular to the direction of cylinders and strips inlateral plane.

The principles of operation of SERS-active structures in optical sensordevices for analyte detection are the same as described in case of FIG.2, and can be easily understood by analogy.

D. Description of Specific Embodiments

This section describes four applications of the optical nanostructuresdescribed above. In these embodiments, which are illustrated in FIGS.12-16, the structure represented by numeral 150 in FIGS. 12 and 13 isthe optical sensor nanostructure described above. For all embodiments,the range of optical nanostructures is intended to encompass the generalstructures described above.

D1. Planar Microfluidic Optical SERS Sensor

In its basic embodiment, the optical structures is used as an opticalsensor for detection of analytes to which the sensor is exposed, e.g.,in a planar microfluidic SERS chip platform that may be used foranalysis of liquid samples with application to disease or environmentalmonitoring. The general schematic diagram of use of a planarmicrofluidic optical SERS sensor with a table top Raman microscope isillustrated on FIG. 16. The SERS active structure of present inventionaccording to embodiments as illustrated in FIGS. 1-5 above may beintegrated into each channel of a microfluidic chip 370 which is placedon a motorized translation table 360 and controlled by an electronicdevice 350 through a computer 180. Sample analyte flow through channelsand analyte molecules are adsorbed into SERS-active surface and analyzedin Raman microscope. Light from a light source 300 through a beamsplitter 312 and focusing optics 70 and microscope objective 72 isdirected to a sample on the surface of SERS substrate. Raman signalgenerated in backscattering geometry through optical system is sent todispersive element 330 and spectra are detected by CCD detector 340 andanalyzed in computer 180. In another embodiment of sensor portableversion of Raman spectrometer may be used. This sensor has broad rangeof use including, but not limited to: Environmental monitoring, Genomicsand Proteomics research, DNA analysis, Pharmaceutical and Drug Industry,Agriculture and Food analysis, Biomedical diagnostics, Biodefence,Industrial monitoring, Forensic Analysis etc.

D2. Filter-based Optical SERS Sensor

That embodiment is illustrated the use of SERS-active structure of thepresent invention integrated into a filter based optical SERS sensorwith a planar (12A and B) or nonplanar (12C and D) SERS-active surface.The SERS-active structure, indicated at 150, is integrated into porousfilters made of optically transparent material such as porous silica inplanar architecture as illustrated by FIGS. 12A and 12B. Filters fromoptically transparent porous silica may be the best for this sensor.Diameter of pores 190 may be in a range of 1-100 microns, depending onthe purpose of the filter. The nanostructure 150 may be integrated intoporous silica by coating pores by silver layer using electrolessdeposition method and subsequent functionalization of silver surface bynanoparticle as described in Example 1 and 2. A non-planar arrangementof pores covered with an SERS-active surface 150 is shown in FIG. 12Cand D. With an analyte solution flowing through the filter, a lasersystem with spectrometer can be used for continuous monitoring ofcontaminants in solution or water 200 flowing through the pores of thefilter 190. That is, the intended application is for continuousmonitoring of contaminants and hazardous materials in a fluid system,such as a water supply system.

D3. Fiber Based Optical SERS Sensor

An application of the invention to a fiber optic sensor is illustratedby FIGS. 13A and B. Here the SERS-active structure 150 is integratedinto a sensor probe 240 which is connected by an imaging fiber 232 (thatcontains between 1000 and 1000,000 of individual fibers fused togetherinto single bundle) with a multichannel Raman analysis system 170.Excitation light from a light source 220 through fiber 232 is deliveredto the SERS-active surface. Water with target analyte flows through achannel having an inflow 200 a flow in and outflow 200 b. Contaminantsin the flow-through water are adsorbed to surface 150 and detected byenhanced Raman scattering. This type of sensor is particularly usefulfor applications involving monitoring the quality of an aqueousenvironment.

D4. Bead-based Optical SERS Sensor

In still another embodiment, the invention contemplates microbeadscovered by the SERS-active coating of present invention, as illustratedin FIGS. 14A and B. Here, spherical beads 210 formed of polyester or asimilar material and having diameters in the range 3-10 micron arecovered by silver layer 20 by method of vacuum deposition, and thislayer in turn is covered by a dielectric layer 30 having thickness in arange 2-40 nm. The coated bead is then covered by NPs 100 which havediameter in a range 50-150 nm. As shown, the NPs are also covered bydielectric coating 30. SERS-active beads can be used as a suspension ina microfluidic optical sensor device or in application requiringaerosols.

The use of SERS-active beads in a microfluidic optical sensor isillustrated by FIG. 14B. Sample analyte in solution is injected througha channel 202 and suspension of SERS-active beads, through a channel200. In mixing chamber 250 analyte is mixed with beads, and analytemolecules are adsorbed onto the surface of beads. In detection area 260analyte is detected by SERS.

SERS-active beads in form of aerosol may be used for distant detectionof warfare biological and chemical agents and explosives as illustratedby FIG. 15. here an aerosol of SERS-active beads 290 is injected from aninjector 280 into a cloud 270 of gas to be analyzed. Analyte and beadsare mixed in the cloud and analyte is adsorbed onto surface ofSERS-active beads. Following this, the beads are collected, e.g., by agas filter, or may be analyzed in situ by a Raman system 300 for exampleRaman LIDAR.

From the foregoing, it can be appreciated how various objects andfeatures of the invention have been met. Model SERS plates constructedin accordance with the invention were prepared and tested with differentRaman systems using adenosine molecule as analyte. A comparison of theresults with that for a commercially available SERS plates and with anIntel porous silica covered by silver SERS plates demonstrates anamplification better at least 6 orders of magnitude over these prior artstructures. The results are robust and reproducible, in that the sameresults were obtained on multiple different set nanostructures over aperiod of several months. The nanostructure plates are stable, sincethey sustain SERS activity for at least 3 month.

In accordance with the invention, and for the first time, substantialSERS signal in a range up to 7000 counts per second was obtained withnew SERS plates at illumination power as low as 5 microW at sample andin some cases even 0.4 microWatt with R6G. This level of signal iscomparable or better than that achieved in sensors based on luminescentdetection; however required illumination power is at least 3 orders ofmagnitude less. Assuming an amplification factor of Intel substrates ina range 10⁶ -10⁸, one can estimate an amplification factor for SERSplates of present invention 10¹²-10¹⁴.

The following examples illustrate various methods of forming and usingthe nanostructures of the invention, but are in no way intended to limitthe scope of the invention.

EXAMPLE 1 Preparation of a Silver-silver Particle Nanostructure bySelf-assembly of Ag Nanoparticles

For each of a number of slides, a silver mirror was deposited on a cleanglass microscope slide by thermal evaporation of the silver (99.995%)using vacuum deposition system (E302, Edwards). The slides were immersedin a 1% aqueous polylysine solution for one hour, forming a polylysinedielectric layer over the silver film. Following rinsing in copiousamount of water, the slides were exposed overnight to a silvernanoparticle suspension of optical density 5 at extinction maximum of450 nm. The self-assembly of the silver particles on the surfaceresulted in the yellow hue (appearance) of the mirrors. The slides werethen rinsed with water and exposed to different analytes for varioustime periods. After the adsorption of analyte molecules slides wereinterrogated with Raman spectrometer yielding SERS spectra.

In the second example, silver nanoparticles were adsorbed on the surfaceof the mirror using poly(vinylpyridine) as the surface modifier (formingthe dielectric layer). Poly(vinylpyridine) was adsorbed on the mirroredsilver and gold surfaces from 1% ethanolic solutions for duration ofseveral hours.

EXAMPLE 2 Preparation of Samples With Self Assembled Silver ParticleNanostructure by Microcontact Printing

In this example, a method of microcontact printing as disclosed forexample in reference (H. S. Shin, et. al. “Direct patterning of silvercolloids by microcontact printing: possibility as SERS substrate array”,Vibrational Spectroscopy, v. 29, p. 79-82, 2002, H. Fan et at.,“Self-Assembly of Ordered, Robust, Three-Dimensional GoldNanocrystal/Silica Arrays”, Science, 304, 567-571 (2004), was used toform a close-packed array of silver nanoparticles on a silver mirror.

Silver nanoparticles were prepared by method disclosed in Lee P. C.,Meisel, D. J., J. Phys. Chem., 86, p. 3391 (1982), Poly(vinylpyrrolidone) was used as the capping agent. First, silver nitrate (0.2g, Aldrich, 99+%) was dissolved into 3 mlL ethylene glycohol (Aldrich,99.8%). 1 g polyvinyl pyrrolidone (Aldrich, MW ≈40 000) was added into15 mL ethylene glycohol and the mixture was stirred and heated to 197°C. The silver nitrate in ethylene glycohol solution was subsequentlyinjected into heated poly(vinyl pyrrolidone). This reaction mixture wasthen heated at 197° C. for 1 hour. The silver nanoparticles wereprecipitated by centrifugation. Specifically, the reaction mixture wascooled to room temperature, diluted with acetone (about 10 times byvolume), and centrifuged at 4000 rpm for 20 min, with the liquid phasebeing removed using a pipette. The nanoparticles are rinsed with water,and washed with acetone and water for 2-3 times, to remove extrasurfactants/polyvinyl pyrrolidone.

Glass slides used for silver deposition were first cleaned by soaking inNaOH (Aldrich, 99%) solution (0.1M NaOH in 75% ethanol aqueoussolution). After 2 hours, glass slides are washed with ultrapure waterand air-dried. A sliver thin film (thickness=100 nm) was deposited onthe cleaned glass slides by Edwards EB3 e-beam evaporator in 432A. Theobtained glass slides were soaked into 1 wt % poly(vinyl pyridine)(Sigma, Mw ≈37 500) solution. After 4 hours, the slides were rinsed withultrapure water and air-dried. The slides were subsequently placed on ahot plate and baked at 50° C. for 15 minutes.

Silver nanoparticles in hexane solution were carefully dropped ontowater surface, where the hexane spreads on the water surface to form athin oil film. As hexane evaporates, the film surface shrinks until allthe hexane is gone and silver nanoparticles are self-assembled into aclose-packed monolayer.

These silver monolayers were transferred to the slice surface bybringing the slide parallel to the water surface and lightly touchingthe substrate to the nanoparticle film. Multiple layers of silvernanoparticle could be achieved by repeating this process. (shown asfollowed figure)

The slides were baked on a hot plate at 50° C. for 15 minutes.

The method used in slide preparation is similar to method disclosed inH. S. Shin, et. al. “Direct patterning of silver colloids bymicrocontact printing: possibility as SERS substrate array”, VibrationalSpectroscopy, v. 29, p. 79-82, 2002, H. Fan et al., “Self-Assembly ofOrdered, Robust, Three-Dimensional Gold Nanocrystal/Silica Arrays”,Science, 304, 567-571, 2004.

An AFM topographic image of typical SERS substrate prepared by thisprotocol is presented in FIG. 6, showing a high density array of NP isclose to periodic structure.

EXAMPLE 3 Experimental Measurements on the Analyte Rhodamine 6 G (R6G)

The experimental system set up used in present experiments is shown onFIG. 7A-7C. Measurements were carried using Horiba-Jobin Yvon Ramanmicroscope LabRam HR 800.

Measurement of SERS spectra from liquid samples was carried out using afluidic cell made from borosilicate glass. A schematic diagram of afluidic cell is presented on FIG. 7A (top view) and in FIG. 7B (crosssectional view). A glass fluidic cell contains fluidic a channel 84formed on a glass slide 14 to a depth of about 1.2 to 2.0 mm. Thethickness' of the SERS-active structure 150 was 0.8 mm. During theexperiment, the optimal value of parameters such as depth of fluidicchannel was determined, e.g., the best conditions for focusing of thelaser light beam through the confocal objective in Raman microscope. Useof the glass cover slip 16 was critical in order to maintain the samethickness of analyte layer during all sets of measurements. As a resultan optimum depth of fluidic channel of about 1.5 mm was determined.

Use of the fluidic cell also allows for determining an accuratedetection limit for analytes in solution, in terms of concentration ofanalyte molecules in solution measured in units of mole/liter. For thatpurposes a Langmuir adsorption isotherm was determined for each analyte.

Aqueous solutions of Rhodamine 6 G (R6G) were prepared in a range ofconcentrations from lowest 10⁻¹⁰ moles/l up to 10⁻³ moles/l. As a firststep, measurements of Raman spectra were taken from solutions with thelowest concentration of analyte and subsequent measurement were donewith the same SERS-active plate but with increasing concentration ofanalyte. At each step of the procedure, analyte solution was injectedinto the fluidic cell using a pipette 160 a, then covered by glass coverslip. After measurement of the Raman spectra, analyte solution wasreplaced by a new one at higher concentration, and the measurement wasrepeated under conditions of focusing the illumination beam. The focusof the Raman microscope was adjusted to obtain optimal illuminationcondition, and these settings were used for all subsequent measurements.During each next step, solution in fluidic cell was replaced by solutionwith increasing concentration of analyte, using pipette 160 b to removeanalyte solution.

FIG. 7C shows the experimental setup employed in the measurements. Inthis figure, optical sensor nanostructure 150 is irradiated by anoptical beam 70 which is focused by a lens assembly 72. Scattered lightfrom the sample is focused in assembly 72, and directed by abeam-splitter 160 to a multichannel Raman analysis system 170, withspectral analysis carried out on computer 180. This method allows forthe use of the same SERS-active substrate in multiple measurements. Italso allowed for testing the robustness of the substrate.

The results of representative experimental data obtained with R6G usingfluidic cell 1.5 mm deep and Raman microscope Horiba-Jobin Yvon LabRamHR 800 are shown on FIGS. 8 to 11. From these data, the quantitativelimit of detection (LOD) was determined for R6G to be 100 nano M/l. TheLOD was define as the first concentration at which distinctive spectralfeatures of R6G first appeared in Raman spectra.

FIG. 8 show SERS spectra of R6G at concentration 500 nanoM/L, with alaser power at sample of 4.1 μW, integration time 10 sec, wavelength ofexcitation light beam 514 nm, objective 50×/0.45, with the light beamfocused on the surface of the substrate, and a diameter of focal spot atsample of 2 micron. This spectra was obtained without subtraction ofbackground. It can be seen that that even at very low illuminationpower, Raman signal is very strong, yielding 7000 counts per second forstrongest lines.

FIG. 9 shows SERS spectra of R6G at the same conditions and set up asfor the FIG. 8 experiment, except that the excitation power at thesample was extremely small, as low as 0.4 μW. Although the Raman signalis less in this case (about 200 counts per second), the signal to noiseratio that characterizes a quality SERS spectra is still is very high,more than 100.

FIGS. 10A and 10B show Raman spectra images obtained by mapping a 20micron by 20 micron area of SERS substrate. Excitation power in thisexperiment was 32 μW, collection time for each individual spectra was 1second, mapped area was 20×20 micron, and the measurement of map wasdone with a 1 micron step and a total number of spectra was 400 points.The whole map was done for 7 min using automated motorized table systemof Horiba-Jobin Yvon LabRam HR 800 Raman Microscope.

FIG. 10A shows a Raman image for intensity of main peak of R6G,integrated over the interval 1280-1400 cm⁻¹ with baseline correction,where intensity is given in %. The results show high uniformity acrossthe surface of enhancement properties of SERS-substrate according topresent invention. Maximum variation of intensity of major spectralfeature is less than 25% as illustrated by FIG. 10B, where spectra withmaximum and minimum intensity are presented for comparison.

FIGS. 11A and 11B show the same data, but where the set of SERS spectraare along one line consisting of 20 points, presented as a 3-D plot inFIG. 11B.

The data demonstrate high uniformity across the surface of theenhancement properties of SERS-substrate in the present invention,meaning that a high density of “hot spot” that is critical for practicaluse of SERS-substrate is achieved, and shows the superiority of thisSERS-substrate over others available in prior art.

in particular, it has been discovered that substrates prepared by thepresent invention have unusually strong enhancement of Raman signalcompared with other SERS substrates. Most impressive is the fact that astrong Raman signal is achieved even at 0.4 microWatt of illuminationpower (See data presented on FIG. 9). Experimental data show that SERSplates of the present invention exceed the amplification of Ramansignals achievable in the currently existing SERS plates developed byIntel Precision Biology Group (S. Chan et al., “Surface Enhanced RamanScattering of Small Molecules from Silver-coated silicon nanopore”,Advanced Materials, 15, 1595-1598, 2003, at least 5 to 6 orders ofmagnitude. This means that the substrate of present invention canprovide a reproducible and stable amplification factor up to 10¹² to10¹⁴, where allowing for single molecule sensitivity.

While the invention has been described with respect to certainembodiments and applications, it will be appreciated how variousmodifications and changes, and additional applications can be madewithout departing from the invention.

1. A microbead composition for use in detecting chemical groups in ananalyte with an amplification factor of at least 10¹⁰ comprising acollection of microbeads, each composed of (a) a microbead substrate;(b) a plasmon resonance mirror formed on the surface of the substrate,(c) disposed over said mirror, a dielectric layer having a thickness of2-40 nm, and (d) a plasmon resonance particle layer composed of aperiodic array of plasmon resonance particles in a selected size rangebetween 50-200 nm and having a coating effective to bind analytemolecules, wherein binding of analyte to the microbeads, and analysis ofthe microbeads and bound analyte by Surface Enhanced Raman Scattering iseffective to produce a Raman spectrum of the chemical groups in theanalyte with an amplification factor of at least 10¹⁰.
 2. The microbeadcomposition of claim 1, wherein microbead substrates have a diameter inthe range 3-10 microns.
 3. The microbead composition of claim 1, whereinthe plasmon resonance particles are substantially uniformly distributedover the surface of the mirror coating on the microbead substrate. 4.The microbead composition of claim 3, wherein the plasmon resonanceparticles are also coated with a dielectric layer.
 5. The microbeadcomposition of claim 1, which is effective to produce a Raman spectrumof the chemical groups in the analyte with an amplification factor of atleast 10¹².
 6. The microbead composition of claim 1, wherein the plasmonresonance mirror has a thickness of between about 30 nm to about 500 nm.7. The microbead composition of claim 1, wherein the plasmon resonancemirror is made from a material selected from silver, gold, or aluminum.